The investigation of spectral distributions commonly entails the determination of a peak location in the independent variable and the intensity or area associated with the peak. This intensity is most frequently estimated on the basis of the peak amplitude for crude purposes, but an integral of the feature, e.g. peak, is required for serious quantitative study. Precise interpretation necessitates knowledge of the line shape for the purpose of resolving the spectral feature from background or from inferring the presence of complex structure otherwise unresolved. Thus, the investigation of spectral shape of a particular feature presupposes that the spectrometer does not introduce distortion contributing to the observed line shape.
A number of techniques are known to minimize or avoid spectral distortion. For example, it is common practice to detect echo signals rather than free-induction decay signals to avoid problems introduced by receiver dead times in close proximity to pulses. It is also known that data acquisition, if initiated from the peak of the echo signal, will result in a spectrum in the frequency domain, free from distortion and therefore no frequency dependent phase shift will occur across the spectrum and the absorption mode signal will faithfully represent the true line shape.
In the prior art a "successive approximation" technique is known wherein the transformed spectrum is analyzed to search for a frequency dependent phase correction, .DELTA..phi.(.omega.) There is an implied time interval, .+-..DELTA.t, by which the data acquisition is advanced/retarded for another, subsequent data acquisition. This technique may require repetition of the experiment, an extremely time consuming approach when weak signals are observed and many excitations are required to produce an acceptable averaged s/n parameter. Alternatively, the data set acquired to represent the echo signal may be truncated by a small number of points provided the remaining data set spans signal. An example of this technique is to be found in Davis, et al, Chem. Phys. Lett., V. 42, pp. 390-394 (1976).
In a variation of this approach, post acquisition processing may include a simple shift of the time origin through linear interpolation of the original data set. Ronemus, et al., J. Mag. Res., V. 70, pp 416-426 (1986).
Another approach practiced in the prior art is to begin data acquisition early with respect to the echo peak and adjust the data set by associating a particular data point with the time origin corresponding to the echo peak. It is necessary in this method to sample the waveform with a frequency sufficiently high enough to locate the peak with the desired precision, whereas the sampling rate necessary to characterize the waveform may be much lower. Consequently, a correspondingly high speed (and expensive) ADC will be needed for the high sampling rate, while this may be unnecessary for the characterization of the waveform.